On a Spurious Cloud/PBL Oscillation in CAM5 CAPT Simulations with CLUBB and MG2 Scheme

The Cloud Layers Unified By Binomials scheme (CLUBB) is a higher-order turbulence and cloud parameterization that is found to improve the representation of boundary cloud variation in CAM5 CAPT simulations based on ARM Observations in the Azores from June 2009 to April 2010. However, a spurious cloud/boundary layer oscillation was identified in CAPT simulations with CLUBB. A case study indicates that the oscillation occurs when the boundary layer undergoes a 4-6-hour cycle of growth, decoupling, and replacement. Global screening proves that the oscillation is a common phenomenon in trade cumulus regimes over the world. This study will discuss the mechanism how drizzle-cloud-turbulence interactions in CAM5 with CLUBB causes the spurious oscillation. The findings from this study can help us better understand the drizzle-cloud-turbulence interactions in climate models with a higher-order turbulence scheme and guide the future improvement of this parameterization. Acknowledgement: This work is supported by the ASR Program for the Office of Science of the U.S. DOE. This work was performed under the auspices of the U.S. DOE by LLNL under contract DE-AC52-07NA27344. LLNL-ABS-669677.